This invention relates to a method and apparatus for hot bending a metal pipe, and especially to a method of keeping the heating temperature substantially constant while gradation bending, in which the bending radius is changed gradually at the start and the end of the bending process to produce smooth bends while avoiding abrupt changes in pipe wall thickness.
The prior art includes a method for hot bending a metal pipe, wherein the pipe is heated locally with a circular heater such as induction heater or the like, and where the heated zone is moved relatively to the pipe by means of moving the pipe and/or the heater while bending moment is applied to said heated zone to cause bending, and after which the pipe is cooled in the vicinity of the bend.
But is it not well known how to prevent abrupt changes in pipe wall thickness due to abrupt change of radius of curvature at the start and the end of the bending process when the relative bending radius (i.e., the ratio of bending radius to pipe diameter R/D) is very small. However, it is very important to prevent such abrupt changes in pipe wall thickness because of problems that make the bending itself very difficult, e.g., swelling or wrinkling at the start of the bending process, and severe concentration of bending stress.
In relation to the method to make said change of pipe wall thickness gentle and smooth, a Japanese Patent Application No. 51-150809 has been laid open in which the bending radius is changed gradually at the start and the end of the bending process and in which the mean radius of bending is made equal to the desired radius. This process is call "gradation bending".
Gradation bending is based on basic principle of hot bending and covers many cases wherein a pipe to be bent is heated locally with a circular heater such as induction heater or the like and the heated zone is moved relatively to the longitudinal direction of the pipe by means of moving the pipe to be bent and/or the heater while a bending moment is applied to the heated zone to cause bending. After bending, the pipe is cooled in the vicinity of the bend. Further, bending may be started at a larger radius than the desired radius and reduced gradually until it becomes slightly smaller than the specified radius within a certain predetermined small range of bending angle.
In a case in which heating temperature changes significantly as a function of the relative speed of the heated zone to the pipe to be bend. Such change can happen in the case of typical prior art induction bender shown in FIG. 1 where the pipe 1 is fed at a constant speed and heater H is displaced gradually for "gradation bending".
In FIG. 1, 1 is a pipe to be bent, 2 is a bent portion of the pipe, 3 is the center of heated zone where deformation of bending arises, H is a heating means (such as induction heater) equipped with cooling means in one body, 4 is a bending arm which clamps pipe 1 at the top of it and can rotate freely around a center 0, 5 and 6 are guide rollers to guide and support the pipe 1 against the bending forces, P is the thrust to feed pipe 1 and exert bending moment at the heated zone 3, W is the speed of pipe 1 to the right, h is the speed of heater H to the left, and A is a point which is an intersection of the axis of pipe 1 and a plane which is vertical to pipe 1 and includes the point 0.
In normal bending, heater H is located at point A or in the vicinity of it and then radius of bending is kept substantially equal to the effective length Ro of bending arm 4.
In the case of gradation bending, heater H is first located at point 3 of FIG. 1 spaced from A by certain proper distance towards bending arm 4 and is displaced gradually to point A in order to change the radius of bending from large to small gradually.
With reference to FIG. 1, the change of bending radius R is accomplished as follows:
Within a minute interval of time .DELTA.t, the pipe 1 is fed to the right by a minute length dS.sub.1 at a constant speed W, while heater H is moved to the left by a minute length dS.sub.2 and the pipe is bent by a minute angle d.theta. where the length of pipe before and after bending is assumed unchanged. ##EQU1## where dS=dS.sub.1 +dS.sub.2
If heater H is not moved and fixed, then: EQU R=dS.sub.1 /d.theta.=Ro (2)
Formula (2) means that radius of bending R is substantially equal to the effective length of bending arm Ro when the position of the heater is fixed.
From formulas (1) and (2) above: ##EQU2##
Since as dS.sub.1 /dt=W, and dS.sub.2 /dt=h: ##EQU3##
The relative speed V of the heated zone to the pipe is: EQU V=W+h (5)
If for instance, bending is started at a radius R twice as larger as Ro, then from formula (4), ##EQU4##
When heater H is moved at a high speed, heating temperature becomes very low if heating power is kept constant. If doubled effective heating power would be supplied, then the heating temperature should be kept substantially constant.
It is normal to control heating temperature by means of controlling heating power corresponding to a deviation of heating temperature measured with an instrument, but such feedback method is not effective when the change of h (or V) is very large.
The present invention is directed to a program for keeping heating temperature substantially constant by controlling the heating power supply or alternatively keeping the relative speed V constant from the start to the end of bending by means of controlling W and h separately.